Conventionally, when retrieving an image, a keyword related to the image is generally used. As a means for retrieving an image which is intuitively similar to a query image which is used as a key of retrieval, there has been provided a method of extracting feature quantities, such as the color, texture, and outline of the query image, from the query image, and retrieving an image on the basis of the feature quantities. Furthermore, there has been proposed a method of dividing the query image into regions, extracting image characteristic quantities from each of the regions, and calculating degrees of similarity between the query image and images to be retrieved.
In general, a portion including an object to be shot and a background portion exist in a captured image. In a case in which feature quantities are extracted from the whole image, the feature quantities of the object to be shot and the feature quantities of the background coexist in the extracted feature quantities. Therefore, especially in a case in which the background portion occupies a large region in the image, the feature quantities of the background become a factor to lower the retrieval accuracy remarkably.
In order to solve this problem, patent reference 1 proposes a method of dividing an image into some regions, extracting feature quantities, and carrying out similarity calculation.
[Patent reference 1] JP,2004-21430,A
However, in accordance with the method disclosed in patent reference 1, an image is divided into rectangular regions, and no method of specifying the region of the object to be shot is disclosed explicitly. Many kinds of feature quantities, such as color, shape, and texture, are extracted from the image, and show different features of the image. Because feature quantities which should be thought as important in the calculation of similarity vary dependently upon images to be retrieved and the user's taste, desired results are hard to be obtained. A further problem is that in a case in which the number of images to be referred to is huge and the similarity calculation is carried out for each of all of many feature quantities, large amounts of computations are needed, the cost of the retrieval system is increased, and the user's convenience may be spoiled.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an image retrieval method and an image retrieval apparatus which achieve improvement in the speed of extraction of feature quantities and similarity calculation by making mutual use of the feature quantities, and which improve the retrieval accuracy and speed by carrying out region division adaptively and selecting an appropriate feature quantity.